In the past few months we got a lot of topics about oil, trans fluid, PS fluid and so on...

I will do some research about some of the FAQ's and paste the link in this thread. If some of you know or read some good thread about maintenance please PM me with the link of the thread. I will sticky and lock this thread for futher SEARCH.

Please use this information wisely. It is compiled from various manufacturers, aftermarket recommendations, personal experiences and the Society of Automotive Engineers. No claim is made as to its accuracy, and as always, don't blindly trust something you see or read on the internet. Your owners manual and vehicles Technical Service Bulletins are what you should follow, but hopefully this will sum it up a little and give some explanation as to what the different services mean.

Editor: mitsuguy
Contributors: mitsuguy, ArmyOfOne

Maintenance Items:

Disclaimer: Regardless what you read here, your owners manual is the absolute source for maintenance intervals and recommendations. This is just meant to give a brief explanation of the basic maintenance services that many vehicles require. It never hurts to do maintenance services early.

Spark Plugs
Gasoline engines need three things to run - fuel, compression (of air or a gas containing oxygen) and spark, these take care of the spark.

Typical Service Interval: 30k, 60k, 90, 100k, 110k, 120k
Typically, if a vehicle is equipped with copper spark plugs, it has a service interval of 30k. Double Platinum (platinum on both ground strap and electrode) tipped plugs have a 60k service interval. Iridium Plugs have 90k to 110k service intervals. In most applications, these are interchangeable and the main consideration is effective service life. In general, Platinum and Iridium tipped plugs cannot be gapped, however copper plugs can. Simply cleaning plugs does little to help them and if you are going to pull them to inspect them, you might as well replace them at the same time.

Why do we replace/service:
Worn spark plugs can cause a decrease in fuel economy, misses in the ignition system and leave raw fuel unburned in the exhaust.

What happens if we don’t service:
Left unchecked, raw fuel in the exhaust caused by misses in the ignition system will cause premature catalytic converter failure.

General Recommendations:
Iridium and Platinum plugs can be well worth the added cost due to their longer service life. Although they may cost 4-6x more than copper plugs, many feel that the longer service life and less labor costs far outweighs the additional cost. Others who do the labor themselves, or have forced induction may prefer the less expensive copper plugs and change them more often.

Coolant
Flows from the engine to the radiator and back again, transferring heat from the engine to the ambient air. Protects internal engine components.

Service Interval: 30k-150k or 2 years to 6 years
Typical green ethylene glycol coolants are good for 2 years or 30k miles. Other coolants will have longer service intervals. Use a hygrometer to test for freeze/boilover point. Add, remove or replace as needed to ensure proper protection. A voltmeter with the positive lead in the coolant (open the radiator cap) and the negative lead to an engine ground will show voltage. If the voltage is more than about one half of a volt, then there is a voltage problem and most likely a galvanic reaction is occurring in the coolant system.

Why do we replace/service:
Coolants have corrosion inhibitors to protect and lubricate the water pump, thermostat and other materials in the cooling system. Over time, these wear out.

What happens if we don’t service:
As coolant wears out, corrosion inside the system sets in and soft materials such as aluminum start to corrode quickly. Due to dissimilar metals used in different parts of the cooling system, coolant can actually become acidic and even produce an electrical charge like a battery. The most important part of coolant is to protect from freezing and give additional boil over protection as well. This can be tested with a hygrometer to ensure proper protection.

General Recommendations:
Service the system long before it is due and try to avoid mixing different types of coolant. There are many universal long life coolants that work well in most systems.

Service Interval: 15k-100k
Manufacturers vary greatly in service recommendations. Some suggest a drain and fill (only replacing 3 or so of the 9-12 quarts in the system), some a drain and fill with filter replacement, and some a full transmission fluid exchange. On vehicles that have a dipstick, the fluid should be relatively bright red in color and not have a burnt smell. If it is very dark or smells burnt, proceed with caution as noted below.

Why do we replace/service:
Like engine oil, transmission fluid breaks down over time, as well as loses its ability to keep the transmission internals clean. It is under extreme heat and pressure.

What happens if we don’t service:
The older the fluid, and the hotter it gets, the less it can do its job of lubricating and cooling your transmission. If your filter is dirty as well, it exponentially increases the problem, by restricting fluid flow, which can overheat your transmission. Essentially, this is just as important of a service as changing the oil and over the life of a car, actually costs less to do than an engine oil change, as the service intervals are much farther between.

General Recommendations:
Keep up with transmission services. Vehicles that suggest complete fluid changes are more important that this takes place on a regular service interval. If you are not sure when the last time your transmission was serviced, and you have it flushed, you may increase your risk of transmission failure. The main problem behind this is that when a dirty transmission is flushed, the detergents in the new fluid may dislodge contaminants leading to stuck solenoids and even more worn clutches. By doing drain and refills every oil change for 3-5 oil changes, you can turn your fluid back to a healthy cherry red color, and gradually remove the contaminants if the complete fluid replacements have not been done regularly.

Differential
Attached to your transmission (in FWD cars) and is in the center of the drive axle in RWD cars. 4WD/AWD vehicles will have differentials in the front and rear, as well as a center differential and/or transfer case.

Service Interval: 12k-30k, some vehicles, never (inspect only)
Service Interval varies greatly depending on the type of driving you do. Under extreme conditions, some vehicles actually call for every 12k miles, while under normal driving conditions, some never suggest fluid replacement. Some FWD vehicles include the differential fluid as part of the transaxle fluid. To inspect, check the fluid level. Breakdown of the fluid may not be apparent, but if the fluid is notably dirty, it should be replaced.

Why do we replace/service:
Similar to engine oil, differential gear oil breaks down over time. It is under high stress and shear loads. As it breaks down, it allows more contact between the metal gears inside the differential. This leads to small metal particles to wear and will wear the gears more and more as time goes on.

What happens if we don’t service:
If services are not kept up with, differentials will wear prematurely. This leads to excess gear noise and will eventually lead to catastrophic failure.

General Recommendations:
Although some manufacturers do not suggest replacement ever, a differential fluid replacement may be cheap insurance to protect a somewhat pricey part. Follow your manufacturers recommendations for fluid weight and pay attention - some require additives such as for limited slip differentials, while others may require synthetic gear oil.

Transfer Case
Transfers the power from the transmission/transaxle to the front and rear axles of a 4WD and many AWD vehicles.

Service Interval: 15k-100k, some vehicles, never (inspect only)
Service Interval varies greatly depending on the type of driving you do. Under extreme conditions, some vehicles actually call for every 15k miles, while under normal driving conditions, some never suggest fluid replacement. To inspect, check fluid levels based on your vehicles configuration. Breakdown of the fluid may not be apparent, but if the fluid is notably dirty, it should be replaced.

Why do we replace/service:
As with any other fluid in a vehicle, the heat and friction created causes the fluid to break down over time.

What happens if we don’t service:
As fluid wears, added wear inside the transfer case is happening as well. A little more complex that differentials, there are moving pieces, sensors and sometimes chains that require proper lubrication.

General Recommendations:
Although some manufacturers do not suggest replacement ever, a transfer case fluid replacement may be cheap insurance to protect a somewhat pricey part. Follow your manufacturers recommendations for fluid weight and pay attention - some may require synthetic gear oil.

Air Filter
Typically in a black box next to the engine, under the hood. Keeps the air entering the engine clean.

Service Interval: 12k-30k
Air filters may become restricted with dust and still look relatively clean. Simply knocking the dust out of it does not help this matter.

Why do we replace/service:
Keeps your engine running as efficiently as possible, the better your engine can take air in, the more efficient it can be.

What happens if we don’t service:
Idle may become rough and random, fuel economy may decrease significantly. The engine may not start or may need excessive cranking to get it started, eventually it can cause problems with the proper air to fuel ratio and cause other failures in the fuel system.

General Recommendations:
Change your air filter every fourth or fifth oil change, perhaps more often if you live in a desert/dusty environment or a large city. Never run without an air filter, with an air filter with a hole in it, or one that does not seal properly - even small dust particles can cause added wear inside an engine and also contaminate the oil. Some manufacturers, such as K&N, offer air filters that never require replacement, only cleaning periodically.

Cabin Air Filter
Also known as an A/C filter - is the equivalent of your home air conditioning units filter, but for your car. Filters the air as it passes through the air conditioning (and heating) system.

Service Interval: 30k-50k
Pretty straightforward here, the more you use the heating and cooling system, the more contaminants are caught here. You can remove the filter and look at it, though flow may be restricted even with a clean appearing filter.

Why do we replace/service:
Just like your home unit, this filter gets dirty and reduces system performance.

What happens if we don’t service:
Air flow through the system is restricted and the system does not work as efficiently. In severe cases, enough resistance to air flow over time can cause the fan motor in the system to burn out.

General Recommendations:
It is nice to breathe clean air and keep the system performing as new. This is a simple way to maintain that.

Service Interval: 15k or more
Society of Automotive Engineers claim that service intervals of 15k miles may be beneficial to the vehicle. Many vehicles have no suggestion for this service at all.

Why do we replace/service:
EGR and PCV systems allow combustion gases and contaminants to enter the intake tract. These are typically dirty, oily gases which accumulate over time on the throttle plate, walls of the intake manifold, injector nozzles and the backside of the intake valves. Servicing cleans the throttle body and adds a detergent into the vacuum line system while the vehicle is running, in order to clean the intake manifold and valves.

What happens if we don’t service:
Over time, the build up causes decreased fuel mileage and can cause odd idle problems and poor power delivery. Left unchecked, other failures in the EGR system and sensors inside the throttle body may cause severe drivability concerns.

General Recommendations:
The better quality of fuel you run, the less you may need to do this service, as valves generally stay cleaner with higher quality gasolines (higher quality, not higher octane). Regardless what type of fuel you use, the gases from the EGR and PCV system will still accumulate on the throttle plate and intake manifold (they are before the injectors).

Service Interval: 3k-15k, 6 months – 1 year
For many years, the general advice was to change the oil every 3k miles or 3 months. With advances in engine and oil technology, these suggested intervals have grown. Many, if not most vehicles, have a suggested service interval of 5k, and some oils suggest service intervals as high as 15k or more. Undeniably, lots of money is spent unnecessarily on oil changes that are not required at the time. Unfortunately, the only way to really tell how much longer oil is safe to use is to send it in for a used oil analysis. The problem with this is that it costs just as much for the analysis as most oil changes cost. It is also suggested to replace oil every 6 months or 1 year as well, regardless mileage. Aside from a used oil analysis, it is impossible to tell how dirty oil is just by its looks. Some oils are very dark even when brand new.

Why do we replace:
Over time, oil breaks down. Some oil will seemingly last forever without breaking down, however over time it becomes contaminated and the detergents and additives are diminished. The contamination is caused by wear of the cylinder walls, bearings, and blow by, which includes combustion gases and unburnt fuel.

What happens if we don’t service:
As the oil breaks down and the additives are not able to keep the oil passageways clean, the oil will start to solidify, or become sludge. This leads to poor lubrication and wear on the engine surfaces – cylinder walls and bearings take the worst of it, and of course, they are also the most important. As time progresses, the engine will start burning more oil and the additional wear on the engine will cause lower oil pressure, which will lead to a catastrophic failure.

General Recommendations:
There are many choices in oil. The type of oil to use is a personal preference. For the most part, so long as oil change intervals are followed, you should have no problems. There are also many different weights of oil. The builder of your car knows best and their suggestions should be followed as far as the weight of oil to use. Many think that higher mileage engines require heavier weight oils. In general, this is not true unless the engine is burning a lot of oil. By using heavier weight oil, you increase oil pressure, but lower the amount of oil that flows through the engine. In modern engines, flow is more important than pressure.

Alignment
How the wheels and tires are positioned and angled, relative to the cars chassis.

Service Interval: 6 months – 2 years
Many manufacturers do not specify an interval for alignments. Many cars are not really well aligned, even from the factory. Some obvious cues for an alignment being required are the car pulling one way or the other, an uneven steering wheel and uneven tire wear. The types of roads, driving style, and quality of roads driven on all have an impact on how often an alignment should be performed.

Why do we replace/service:
To ease the driving experience, minimize tire wear and minimize wear on suspension components.

What happens if we don’t service:
Left unchecked, a bad alignment can cause uneven and accelerated tire wear, costing potentially more than an alignment would have cost. It can also cause wear and undue stress on the vehicles steering and suspension components.

General Recommendations:
It is very hard to get an absolute perfect alignment, but that is ok, it doesn’t have to be absolutely perfect for good tire wear and to avoid a pull one way or the other. Excessive camber (the upper part of the tire leaning in) can lead to tracking the road and wear on the inside edges. Toe is the number 1 tire wear angle and it is very important it get as close as possible to the exact specification. Caster is an angle that only affects the front tires. It is how much the wheel leans back, similar to a motorcycle. Caster is an angle that can cause a car to pull if it is not correct, and excessive caster leads to better high speed stability, but more steering effort.

Tire Pressure / Rotation

Service Interval: 5-6k miles for rotation, Every month for air pressure checks
If you notice uneven tire wear, an up and down pattern along the tread, or outside edgewear, it may be time (or past time) for a tire rotation. With radial tires, and especially with lower profile radial tires, it is difficult to determine air pressure simply by looking at the tires. A tire may look the same whether it has 15 psi or 50 psi in it - the only way to truly check is with a proper air gauge.

Why do we replace/service:
To promote long tire life and prevent uneven wear.

What happens if we don’t service:
Tires may wear unevenly and cause damage to the tires. If a tire is run even a few psi low, it will wear faster, not handle as well, reduce fuel mileage and if ran that way long enough can cause irreparable tire damage. If a regular rotation schedule is not followed, tires may start to cup or feather (when the tread blocks become uneven and have one side higher than the next). This causes excessive noise from the tires. Also, depending on the drive configuration of the car, the drive tires may wear faster.

General Recommendations:
On average, a tire may lose 1 psi per month, and when the temperature changes, may lose 1 psi per 10 degrees change cooler, or vice versa. This can be reduced by using nitrogen in tires, however a tire filled with nitrogen will still lose pressure through typical punctures or leaks and should still be checked monthly. Rotations are not possible with all cars. Vehicles with staggered fitments front and rear may be left to only rotate side to side. This is not very easy with directional tires and on those vehicles, it may be recommended to use asymmetrical tires so that this is possible.

PCV Valve
Located in one or both of the valve covers, allows excess crankcase pressure to circulate back in to the intake to be burned.

Service Interval: 25k, sometimes never (inspect only)
Many may last indefinitely, many do not have suggested service intervals, some vehicles do not have typical PCV valves at all. Most PCV valves typically have a metal ball inside it that will rattle when the PCV valve is not stuck closed. Just because it rattles does not make it good, however.

Why do we replace/service:
Over time, the one way check valve may stick, usually in the closed position. This allows the crankcase pressure to build up.

What happens if we don’t service:
Excess crankcase pressure that builds up can cause multiple oil leaks from many different places. This is probably the #1 cause of oil leaks in car engines today.

General Recommendations:
This is an inexpensive part that can cause lots of money in potential repairs.

Power Steering Fluid
Pumped from a high pressure pump attached to the engine, power steering fluid under pressure assists in turning at lower speeds.

Service Interval: Undetermined, 45k-60k perhaps
Most vehicles never list this as a maintenance item whatsoever, aside from checking the fluid level. Aside from a very dirty appearance, it is difficult to evaluate this fluid by looks or feel.

Why do we replace/service:
Power steering fluid typically has a high amount of detergent and is generally similar or may even be transmission fluid in many vehicles. These detergents wear out and the fluid breaks down over time.

What happens if we don’t service:
Over time, fluid break down can cause premature wear on the power steering components.

General Recommendations:
Many vehicles never call for anything more than a simple fluid level check. Performing a fluid replacement every 45-60k miles may be good insurance against failure of these components.

Brake Fluid Change:
Hydraulic fluid that is moved from the master cylinder (behind your foot when you press the brake pedal) to the braking system at the wheels, typically calipers in the front and calipers (disc) or wheel cylinders (drum) in the rear.

Service Interval: 2 years, or as required when tested
Many manufacturers do not have brake fluid change intervals. Brake fluid deteriorates over time and can be tested for how much metal it has leached from the system. Test strips, which test parts per million of copper, show how much metal has been leeched into the system. Brake fluid is also hygroscopic, which means it absorbs water. Although it is a mostly sealed system, moisture can still be absorbed into the system, making the brake fluids' boiling point much lower, which can result in a very soft pedal under heavy braking.

Why do we replace/service:
Aside from the benefit of bleeding the whole system at the time, returning a better pedal feel, brake fluid becomes corrosive over time. Replacing it replenishes the corrosion inhibitors.

What happens if we don’t service:
It actually can leech copper out of the brake lines, eventually leading to leaks (internal and external) and potential ABS hydraulic unit problems.

General Recommendations:
Another relatively inexpensive service, the gain in pedal feel from bleeding the fluid at the same time is a good thing, as well as preventive maintenance for the system.

Timing Belt
This belt is responsible for keeping the bottom end of the engine (the crank) in time with the upper end of the engine (cam or cams).

Typical Service Interval: 60k-90k
Timing belts may look brand new up until the day the break or cause problems. Mileage is the only real way to tell service interval.

Why do we replace/service:
A timing belt deteriorates over time - it stretches, can crack, and become brittle.

What happens if we don’t service:
If a timing belt breaks or the teeth wear off / strip off, on many modern engines, it means that the valves in the head interfere with the pistons in the block. If this happens while the engine is running, it typically means at minimum, the engine needs torn down, head removed and valves replaced. Some engines are not interference engines, but most are anymore.

General Recommendations:
Timing belts can be a somewhat expensive replacement, but timing belt maintenance is always much cheaper than the repairs that may be required should a belt break while the vehicle is being driven. On many vehicles, the water pump is behind the timing belt, or is driven by the timing belt. Many choose to replace the water pump when the timing belt is being replaced, as there is virtually no additional labor, and, should a water pump fail, the timing belt will have to be replaced, again.

Here is the list and recommendation for the spark plug replacement that list in the Owner's Manual and Repair Manual for Lexus models and years.ES300:
1990-1998 60,000 miles
1999 90,000 miles
2000 to date 120,000 miles

IS300:
2001 to date 120,000 miles
RX300:
1999 to date 90,000 miles

SC400/430
1992-1998 60,000 miles
1999 90,000 miles
2000 to date 120,000 miles

GS300/SC300
1993-2000 60,000 miles
2001 to date 120,000 miles

GS400/430
1998 60,000 miles
1999 90,000 miles
2000 to date 120,000 miles

LS400/430
1990-1998 60,000 miles
1999 90,000 miles
2000 to date 120,000 miles

Please use this information wisely. It is compiled from various manufacturers, aftermarket recommendations, personal experiences and the Society of Automotive Engineers. No claim is made as to its accuracy, and as always, don't blindly trust something you see or read on the internet. Your owners manual and vehicles Technical Service Bulletins are what you should follow, but hopefully this will sum it up a little and give some explanation as to what the different services mean.

Editor: mitsuguy
Contributors: mitsuguy, ArmyOfOne

Maintenance Items:

Disclaimer: Regardless what you read here, your owners manual is the absolute source for maintenance intervals and recommendations. This is just meant to give a brief explanation of the basic maintenance services that many vehicles require. It never hurts to do maintenance services early.

Spark Plugs
Gasoline engines need three things to run - fuel, compression (of air or a gas containing oxygen) and spark, these take care of the spark.

Typical Service Interval: 30k, 60k, 90, 100k, 110k, 120k
Typically, if a vehicle is equipped with copper spark plugs, it has a service interval of 30k. Double Platinum (platinum on both ground strap and electrode) tipped plugs have a 60k service interval. Iridium Plugs have 90k to 110k service intervals. In most applications, these are interchangeable and the main consideration is effective service life. In general, Platinum and Iridium tipped plugs cannot be gapped, however copper plugs can. Simply cleaning plugs does little to help them and if you are going to pull them to inspect them, you might as well replace them at the same time.

Why do we replace/service:
Worn spark plugs can cause a decrease in fuel economy, misses in the ignition system and leave raw fuel unburned in the exhaust.

What happens if we donít service:
Left unchecked, raw fuel in the exhaust caused by misses in the ignition system will cause premature catalytic converter failure.

General Recommendations:
Iridium and Platinum plugs can be well worth the added cost due to their longer service life. Although they may cost 4-6x more than copper plugs, many feel that the longer service life and less labor costs far outweighs the additional cost. Others who do the labor themselves, or have forced induction may prefer the less expensive copper plugs and change them more often.

Coolant
Flows from the engine to the radiator and back again, transferring heat from the engine to the ambient air. Protects internal engine components.

Service Interval: 30k-150k or 2 years to 6 years
Typical green ethylene glycol coolants are good for 2 years or 30k miles. Other coolants will have longer service intervals. Use a hygrometer to test for freeze/boilover point. Add, remove or replace as needed to ensure proper protection. A voltmeter with the positive lead in the coolant (open the radiator cap) and the negative lead to an engine ground will show voltage. If the voltage is more than about one half of a volt, then there is a voltage problem and most likely a galvanic reaction is occurring in the coolant system.

Why do we replace/service:
Coolants have corrosion inhibitors to protect and lubricate the water pump, thermostat and other materials in the cooling system. Over time, these wear out.

What happens if we donít service:
As coolant wears out, corrosion inside the system sets in and soft materials such as aluminum start to corrode quickly. Due to dissimilar metals used in different parts of the cooling system, coolant can actually become acidic and even produce an electrical charge like a battery. The most important part of coolant is to protect from freezing and give additional boil over protection as well. This can be tested with a hygrometer to ensure proper protection.

General Recommendations:
Service the system long before it is due and try to avoid mixing different types of coolant. There are many universal long life coolants that work well in most systems.

Service Interval: 15k-100k
Manufacturers vary greatly in service recommendations. Some suggest a drain and fill (only replacing 3 or so of the 9-12 quarts in the system), some a drain and fill with filter replacement, and some a full transmission fluid exchange. On vehicles that have a dipstick, the fluid should be relatively bright red in color and not have a burnt smell. If it is very dark or smells burnt, proceed with caution as noted below.

Why do we replace/service:
Like engine oil, transmission fluid breaks down over time, as well as loses its ability to keep the transmission internals clean. It is under extreme heat and pressure.

What happens if we donít service:
The older the fluid, and the hotter it gets, the less it can do its job of lubricating and cooling your transmission. If your filter is dirty as well, it exponentially increases the problem, by restricting fluid flow, which can overheat your transmission. Essentially, this is just as important of a service as changing the oil and over the life of a car, actually costs less to do than an engine oil change, as the service intervals are much farther between.

General Recommendations:
Keep up with transmission services. Vehicles that suggest complete fluid changes are more important that this takes place on a regular service interval. If you are not sure when the last time your transmission was serviced, and you have it flushed, you may increase your risk of transmission failure. The main problem behind this is that when a dirty transmission is flushed, the detergents in the new fluid may dislodge contaminants leading to stuck solenoids and even more worn clutches. By doing drain and refills every oil change for 3-5 oil changes, you can turn your fluid back to a healthy cherry red color, and gradually remove the contaminants if the complete fluid replacements have not been done regularly.

Differential
Attached to your transmission (in FWD cars) and is in the center of the drive axle in RWD cars. 4WD/AWD vehicles will have differentials in the front and rear, as well as a center differential and/or transfer case.

Service Interval: 12k-30k, some vehicles, never (inspect only)
Service Interval varies greatly depending on the type of driving you do. Under extreme conditions, some vehicles actually call for every 12k miles, while under normal driving conditions, some never suggest fluid replacement. Some FWD vehicles include the differential fluid as part of the transaxle fluid. To inspect, check the fluid level. Breakdown of the fluid may not be apparent, but if the fluid is notably dirty, it should be replaced.

Why do we replace/service:
Similar to engine oil, differential gear oil breaks down over time. It is under high stress and shear loads. As it breaks down, it allows more contact between the metal gears inside the differential. This leads to small metal particles to wear and will wear the gears more and more as time goes on.

What happens if we donít service:
If services are not kept up with, differentials will wear prematurely. This leads to excess gear noise and will eventually lead to catastrophic failure.

General Recommendations:
Although some manufacturers do not suggest replacement ever, a differential fluid replacement may be cheap insurance to protect a somewhat pricey part. Follow your manufacturers recommendations for fluid weight and pay attention - some require additives such as for limited slip differentials, while others may require synthetic gear oil.

Transfer Case
Transfers the power from the transmission/transaxle to the front and rear axles of a 4WD and many AWD vehicles.

Service Interval: 15k-100k, some vehicles, never (inspect only)
Service Interval varies greatly depending on the type of driving you do. Under extreme conditions, some vehicles actually call for every 15k miles, while under normal driving conditions, some never suggest fluid replacement. To inspect, check fluid levels based on your vehicles configuration. Breakdown of the fluid may not be apparent, but if the fluid is notably dirty, it should be replaced.

Why do we replace/service:
As with any other fluid in a vehicle, the heat and friction created causes the fluid to break down over time.

What happens if we donít service:
As fluid wears, added wear inside the transfer case is happening as well. A little more complex that differentials, there are moving pieces, sensors and sometimes chains that require proper lubrication.

General Recommendations:
Although some manufacturers do not suggest replacement ever, a transfer case fluid replacement may be cheap insurance to protect a somewhat pricey part. Follow your manufacturers recommendations for fluid weight and pay attention - some may require synthetic gear oil.

Air Filter
Typically in a black box next to the engine, under the hood. Keeps the air entering the engine clean.

Service Interval: 12k-30k
Air filters may become restricted with dust and still look relatively clean. Simply knocking the dust out of it does not help this matter.

Why do we replace/service:
Keeps your engine running as efficiently as possible, the better your engine can take air in, the more efficient it can be.

What happens if we donít service:
Idle may become rough and random, fuel economy may decrease significantly. The engine may not start or may need excessive cranking to get it started, eventually it can cause problems with the proper air to fuel ratio and cause other failures in the fuel system.

General Recommendations:
Change your air filter every fourth or fifth oil change, perhaps more often if you live in a desert/dusty environment or a large city. Never run without an air filter, with an air filter with a hole in it, or one that does not seal properly - even small dust particles can cause added wear inside an engine and also contaminate the oil. Some manufacturers, such as K&N, offer air filters that never require replacement, only cleaning periodically.

Cabin Air Filter
Also known as an A/C filter - is the equivalent of your home air conditioning units filter, but for your car. Filters the air as it passes through the air conditioning (and heating) system.

Service Interval: 30k-50k
Pretty straightforward here, the more you use the heating and cooling system, the more contaminants are caught here. You can remove the filter and look at it, though flow may be restricted even with a clean appearing filter.

Why do we replace/service:
Just like your home unit, this filter gets dirty and reduces system performance.

What happens if we donít service:
Air flow through the system is restricted and the system does not work as efficiently. In severe cases, enough resistance to air flow over time can cause the fan motor in the system to burn out.

General Recommendations:
It is nice to breathe clean air and keep the system performing as new. This is a simple way to maintain that.

Service Interval: 15k or more
Society of Automotive Engineers claim that service intervals of 15k miles may be beneficial to the vehicle. Many vehicles have no suggestion for this service at all.

Why do we replace/service:
EGR and PCV systems allow combustion gases and contaminants to enter the intake tract. These are typically dirty, oily gases which accumulate over time on the throttle plate, walls of the intake manifold, injector nozzles and the backside of the intake valves. Servicing cleans the throttle body and adds a detergent into the vacuum line system while the vehicle is running, in order to clean the intake manifold and valves.

What happens if we donít service:
Over time, the build up causes decreased fuel mileage and can cause odd idle problems and poor power delivery. Left unchecked, other failures in the EGR system and sensors inside the throttle body may cause severe drivability concerns.

General Recommendations:
The better quality of fuel you run, the less you may need to do this service, as valves generally stay cleaner with higher quality gasolines (higher quality, not higher octane). Regardless what type of fuel you use, the gases from the EGR and PCV system will still accumulate on the throttle plate and intake manifold (they are before the injectors).

Service Interval: 3k-15k, 6 months Ė 1 year
For many years, the general advice was to change the oil every 3k miles or 3 months. With advances in engine and oil technology, these suggested intervals have grown. Many, if not most vehicles, have a suggested service interval of 5k, and some oils suggest service intervals as high as 15k or more. Undeniably, lots of money is spent unnecessarily on oil changes that are not required at the time. Unfortunately, the only way to really tell how much longer oil is safe to use is to send it in for a used oil analysis. The problem with this is that it costs just as much for the analysis as most oil changes cost. It is also suggested to replace oil every 6 months or 1 year as well, regardless mileage. Aside from a used oil analysis, it is impossible to tell how dirty oil is just by its looks. Some oils are very dark even when brand new.

Why do we replace:
Over time, oil breaks down. Some oil will seemingly last forever without breaking down, however over time it becomes contaminated and the detergents and additives are diminished. The contamination is caused by wear of the cylinder walls, bearings, and blow by, which includes combustion gases and unburnt fuel.

What happens if we donít service:
As the oil breaks down and the additives are not able to keep the oil passageways clean, the oil will start to solidify, or become sludge. This leads to poor lubrication and wear on the engine surfaces Ė cylinder walls and bearings take the worst of it, and of course, they are also the most important. As time progresses, the engine will start burning more oil and the additional wear on the engine will cause lower oil pressure, which will lead to a catastrophic failure.

General Recommendations:
There are many choices in oil. The type of oil to use is a personal preference. For the most part, so long as oil change intervals are followed, you should have no problems. There are also many different weights of oil. The builder of your car knows best and their suggestions should be followed as far as the weight of oil to use. Many think that higher mileage engines require heavier weight oils. In general, this is not true unless the engine is burning a lot of oil. By using heavier weight oil, you increase oil pressure, but lower the amount of oil that flows through the engine. In modern engines, flow is more important than pressure.

Alignment
How the wheels and tires are positioned and angled, relative to the cars chassis.

Service Interval: 6 months Ė 2 years
Many manufacturers do not specify an interval for alignments. Many cars are not really well aligned, even from the factory. Some obvious cues for an alignment being required are the car pulling one way or the other, an uneven steering wheel and uneven tire wear. The types of roads, driving style, and quality of roads driven on all have an impact on how often an alignment should be performed.

Why do we replace/service:
To ease the driving experience, minimize tire wear and minimize wear on suspension components.

What happens if we donít service:
Left unchecked, a bad alignment can cause uneven and accelerated tire wear, costing potentially more than an alignment would have cost. It can also cause wear and undue stress on the vehicles steering and suspension components.

General Recommendations:
It is very hard to get an absolute perfect alignment, but that is ok, it doesnít have to be absolutely perfect for good tire wear and to avoid a pull one way or the other. Excessive camber (the upper part of the tire leaning in) can lead to tracking the road and wear on the inside edges. Toe is the number 1 tire wear angle and it is very important it get as close as possible to the exact specification. Caster is an angle that only affects the front tires. It is how much the wheel leans back, similar to a motorcycle. Caster is an angle that can cause a car to pull if it is not correct, and excessive caster leads to better high speed stability, but more steering effort.

Tire Pressure / Rotation

Service Interval: 5-6k miles for rotation, Every month for air pressure checks
If you notice uneven tire wear, an up and down pattern along the tread, or outside edgewear, it may be time (or past time) for a tire rotation. With radial tires, and especially with lower profile radial tires, it is difficult to determine air pressure simply by looking at the tires. A tire may look the same whether it has 15 psi or 50 psi in it - the only way to truly check is with a proper air gauge.

Why do we replace/service:
To promote long tire life and prevent uneven wear.

What happens if we donít service:
Tires may wear unevenly and cause damage to the tires. If a tire is run even a few psi low, it will wear faster, not handle as well, reduce fuel mileage and if ran that way long enough can cause irreparable tire damage. If a regular rotation schedule is not followed, tires may start to cup or feather (when the tread blocks become uneven and have one side higher than the next). This causes excessive noise from the tires. Also, depending on the drive configuration of the car, the drive tires may wear faster.

General Recommendations:
On average, a tire may lose 1 psi per month, and when the temperature changes, may lose 1 psi per 10 degrees change cooler, or vice versa. This can be reduced by using nitrogen in tires, however a tire filled with nitrogen will still lose pressure through typical punctures or leaks and should still be checked monthly. Rotations are not possible with all cars. Vehicles with staggered fitments front and rear may be left to only rotate side to side. This is not very easy with directional tires and on those vehicles, it may be recommended to use asymmetrical tires so that this is possible.

PCV Valve
Located in one or both of the valve covers, allows excess crankcase pressure to circulate back in to the intake to be burned.

Service Interval: 25k, sometimes never (inspect only)
Many may last indefinitely, many do not have suggested service intervals, some vehicles do not have typical PCV valves at all. Most PCV valves typically have a metal ball inside it that will rattle when the PCV valve is not stuck closed. Just because it rattles does not make it good, however.

Why do we replace/service:
Over time, the one way check valve may stick, usually in the closed position. This allows the crankcase pressure to build up.

What happens if we donít service:
Excess crankcase pressure that builds up can cause multiple oil leaks from many different places. This is probably the #1 cause of oil leaks in car engines today.

General Recommendations:
This is an inexpensive part that can cause lots of money in potential repairs.

Power Steering Fluid
Pumped from a high pressure pump attached to the engine, power steering fluid under pressure assists in turning at lower speeds.

Service Interval: Undetermined, 45k-60k perhaps
Most vehicles never list this as a maintenance item whatsoever, aside from checking the fluid level. Aside from a very dirty appearance, it is difficult to evaluate this fluid by looks or feel.

Why do we replace/service:
Power steering fluid typically has a high amount of detergent and is generally similar or may even be transmission fluid in many vehicles. These detergents wear out and the fluid breaks down over time.

What happens if we donít service:
Over time, fluid break down can cause premature wear on the power steering components.

General Recommendations:
Many vehicles never call for anything more than a simple fluid level check. Performing a fluid replacement every 45-60k miles may be good insurance against failure of these components.

Brake Fluid Change:
Hydraulic fluid that is moved from the master cylinder (behind your foot when you press the brake pedal) to the braking system at the wheels, typically calipers in the front and calipers (disc) or wheel cylinders (drum) in the rear.

Service Interval: 2 years, or as required when tested
Many manufacturers do not have brake fluid change intervals. Brake fluid deteriorates over time and can be tested for how much metal it has leached from the system. Test strips, which test parts per million of copper, show how much metal has been leeched into the system. Brake fluid is also hygroscopic, which means it absorbs water. Although it is a mostly sealed system, moisture can still be absorbed into the system, making the brake fluids' boiling point much lower, which can result in a very soft pedal under heavy braking.

Why do we replace/service:
Aside from the benefit of bleeding the whole system at the time, returning a better pedal feel, brake fluid becomes corrosive over time. Replacing it replenishes the corrosion inhibitors.

What happens if we donít service:
It actually can leech copper out of the brake lines, eventually leading to leaks (internal and external) and potential ABS hydraulic unit problems.

General Recommendations:
Another relatively inexpensive service, the gain in pedal feel from bleeding the fluid at the same time is a good thing, as well as preventive maintenance for the system.

Timing Belt
This belt is responsible for keeping the bottom end of the engine (the crank) in time with the upper end of the engine (cam or cams).

Typical Service Interval: 60k-90k
Timing belts may look brand new up until the day the break or cause problems. Mileage is the only real way to tell service interval.

Why do we replace/service:
A timing belt deteriorates over time - it stretches, can crack, and become brittle.

What happens if we donít service:
If a timing belt breaks or the teeth wear off / strip off, on many modern engines, it means that the valves in the head interfere with the pistons in the block. If this happens while the engine is running, it typically means at minimum, the engine needs torn down, head removed and valves replaced. Some engines are not interference engines, but most are anymore.

General Recommendations:
Timing belts can be a somewhat expensive replacement, but timing belt maintenance is always much cheaper than the repairs that may be required should a belt break while the vehicle is being driven. On many vehicles, the water pump is behind the timing belt, or is driven by the timing belt. Many choose to replace the water pump when the timing belt is being replaced, as there is virtually no additional labor, and, should a water pump fail, the timing belt will have to be replaced, again.

Thank you very much for this post here. Great information was given because I use to be one of those people that would wonder what exactly would happen if I didn't get my tires, service, etc. done at a certain mileage/timeframe, what would happen. This has helped me out. Printing this out now, so I have no excuse not to stop by Kaufmann Tires this weekend and get my tires rotated, balanced, an aligned. Thanks again!